Hybrid ball bearing pair and a compressor bearing arrangement

ABSTRACT

A pair of hybrid ball bearings have stainless steel rings and ceramic balls and are configured such that when the first inner ring and the first outer ring are in contact with the first plurality of ceramic balls under zero measuring load and zero mounting load and the second inner ring and the second outer ring are in contact with the second plurality of ceramic balls under zero measuring load and zero mounting load a certain gap exists between either the inner rings or the outer rings. The gap is based on the diameter of the inner ring or on the pitch diameter of the balls and on whether the bearings have an X configuration or an O configuration.

CROSS-REFERENCE

The present application is a continuation of U.S. Ser. No. 15/962,316, filed Apr. 25, 2018, the entire contents of which are hereby incorporated by reference.

TECHNOLOGICAL FIELD

The invention relates to two hybrid ball bearings and a compressor bearing arrangement.

BACKGROUND

For example, in the area of refrigerant compressors, it is known in principle to supply the refrigerant itself as an ultra-low-viscosity lubricant to rolling bearings of the compressor in the sense of lubrication with ultra-pure lubricating film thickness. Apart from the liquid impurities usually present in the refrigerant, this can be a pure refrigerant lubrication. In a modification, one or more other particularly lubrication-relevant substances may be added to the refrigerant, for example, in total up to a concentration of maximum 1%. Further some of these compressors are operated at comparatively high speeds number of revolutions. So, the optimization of the bearing life time is subject to several constraints.

SUMMARY

Two hybrid ball bearings have at least one ring-shaped inner raceway element, on which balls of at least one of the two hybrid ball bearings are rolling off, with an inner diameter, whereby the two hybrid ball bearings are configured to provide an axial clearance with zero measuring and zero mounting loads as follows, if the two hybrid ball bearings are arranged face-to-face or back-to-back:

The axial clearance is between −5 and 5 or 25 and 40 μm, if the inner diameter is smaller or equal to 80 mm,

the axial clearance is between −5 and 20 μm, if the inner diameters is greater than 80 mm and smaller than or equal to 180 mm and,

the axial clearance is between −5 and 30 μm, if the inner diameters is greater than 180 mm.

Two hybrid ball bearings which are configured to provide following axial clearance with zero measuring and zero mounting loads, if the two hybrid ball bearings are arranged face-to-face or back-to-back:

The axial clearance is between −5 and 5 or 25 and 40 μm, if the pitch diameter of one of the two hybrid ball bearings is smaller or equal to 100 mm,

the axial clearance is between −5 and 20 μm, if the pitch diameter of one of the two hybrid ball bearings is greater than 100 mm and smaller than or equal to 200 mm and,

the axial clearance is between −5 and 30 μm, if the pitch diameter of one of the two hybrid ball bearings is greater than 200 mm.

A compressor bearing arrangement has two hybrid ball bearings for a rotatable support of a rotor of the compressor versus a stator of the compressor, whereby the two hybrid ball bearings are arranged face-to-face or back-to-back with following axial clearance with zero measuring and zero mounting loads:

The axial clearance is between −5 and 5 or 25 and 40 μm, if an inner diameter of a ring-shaped inner raceway element of at least one of the two hybrid ball bearings is smaller or equal to 80 mm or if a pitch diameter of one of the two hybrid ball bearings is smaller or equal to 100 mm,

the axial clearance is between −5 and 20 μm, if an inner diameter of a ring-shaped inner raceway element of at least one of the two hybrid ball bearings is greater than 80 mm and smaller than or equal to 180 mm or if a pitch diameter of one of the two hybrid ball bearings is greater than 100 mm and smaller than or equal to 200 mm and,

the axial clearance is between −5 and 30 μm, if an inner diameter of a ring-shaped inner raceway element of at least one of the two hybrid ball bearings is greater than 180 mm or if a pitch diameter of one of the two hybrid ball bearings is greater than 200 mm.

Because axial clearance can also be interpreted as a gap between inner bearing rings when the outer bearing rings are in direct contact and as a gap between outer bearing rings when inner bearing rings are in direct contact, an aspect of the disclosure can also be described as a pair of hybrid ball bearings comprising a first hybrid ball bearing comprising a first stainless steel inner ring having an inner diameter, a first stainless steel outer ring and a first plurality of ceramic balls between the first inner ring and the first outer ring and a second hybrid ball bearing comprising a second stainless steel inner ring having the inner diameter, a second stainless steel outer ring and a second plurality of ceramic balls between the second inner ring and the second outer ring. The pair of hybrid ball bearings are configured a) such that when the first inner ring and the first outer ring are in contact with the first plurality of ceramic balls under zero measuring load and zero mounting load and the second inner ring and the second outer ring are in contact with the second plurality of ceramic balls under zero measuring load and zero mounting load, and the first inner ring is in direct contact with the second inner ring, a first gap exists between the first outer ring and the second outer ring or b) such that when the first inner ring and the first outer ring are in contact with the first plurality of ceramic balls under zero measuring load and zero mounting load and the second inner ring and the second outer ring are in contact with the second plurality of ceramic balls under zero measuring load and zero mounting load, and the first outer ring is in direct contact with the second outer ring, a second gap exists between the first inner ring and the second inner ring. The first gap and the second gap are related to the inner diameter as follows: if the pair of hybrid ball bearings have an X configuration and the inner diameter is less than or equal to 80 mm, the first gap is less than 5 μm or the second gap is less than 5 μm or the second gap is between 25 μm and 40 μm; if the pair of hybrid ball bearings have the X configuration and the inner diameter is greater than 80 mm and less than or equal to 180 mm, the first gap is less than 5 μm or the second gap is less than 20 μm; if the pair of hybrid ball bearings have the X configuration and the inner diameter is greater than 180 mm, the first gap is less than 5 μm or the second gap is less than 30 μm; if the pair of hybrid ball bearings have an O configuration and the inner diameter is less than or equal to 80 mm, the second gap is less than 5 μm or the first gap is less than 5 μm or the first gap is between 25 μm and 40 μm; if the pair of hybrid ball bearings have the O configuration and the inner diameter is greater than 80 mm and less than or equal to 180 mm, the second gap is less than 5 μm or the first gap is less than 20 μm; and if the pair of hybrid ball bearings have the O configuration and the inner diameter is greater than 180 mm, the second gap is less than 5 μm or the first gap is less than 30 μm.

A further aspect of the disclosure comprises a pair of hybrid ball bearings including a first hybrid ball bearing comprising a first stainless steel inner ring having a pitch diameter, a first stainless steel outer ring and a first plurality of ceramic balls between the first inner ring and the first outer ring and a second hybrid ball bearing comprising a second stainless steel inner ring having the pitch diameter, a second stainless steel outer ring and a second plurality of ceramic balls between the second inner ring and the second outer ring. The pair of hybrid ball bearings are configured a) such that when the first inner ring and the first outer ring are in contact with the first plurality of ceramic balls under zero measuring load and zero mounting load and the second inner ring and the second outer ring are in contact with the second plurality of ceramic balls under zero measuring load and zero mounting load, and the first inner ring is in direct contact with the second inner ring, a first gap exists between the first outer ring and the second outer ring or b) such that when the first inner ring and the first outer ring are in contact with the first plurality of ceramic balls under zero measuring load and zero mounting load and the second inner ring and the second outer ring are in contact with the second plurality of ceramic balls under zero measuring load and zero mounting load, and the first outer ring is in direct contact with the second outer ring, a second gap exists between the first inner ring and the second inner ring. The first gap or the second gap is related to the pitch diameter as follows: if the pair of hybrid ball bearings have an X configuration and the pitch diameter is less than or equal to 100 mm, the first gap is less than 5 μm or the second gap is less than 5 μm or the second gap is between 25 μm and 40 μm; if the pair of hybrid ball bearings have the X configuration and the pitch diameter is greater than 100 mm and less than or equal to 200 mm, the first gap is less than 5 μm or the second gap is less than 20 μm; if the pair of hybrid ball bearings have the X configuration and the pitch diameter is greater than 200 mm, the first gap is less than 5 μm or the second gap is less than 30 μm; if the pair of hybrid ball bearings have an O configuration and the pitch diameter is less than or equal to 100 mm, the second gap is less than 5 μm or the first gap is less than 5 μm or the first gap is between 25 μm and 40 μm; if the pair of hybrid ball bearings have the O configuration and the pitch diameter is greater than 100 mm and less than or equal to 200 mm, the second gap is less than 5 μm or the first gap is less than 20 μm; and if the pair of hybrid ball bearings have the O configuration and the pitch diameter is greater than 200 mm, the second gap is less than 5 μm or the first gap is less than 30 μm.

Extensive series of investigations were conducted. These investigations were carried out for various sets of two hybrid ball bearings in X- and O-arrangements for a high rotation speed (between 2500 and 25000 rpm or ndm-values being between 500000 and 1200000 mm/60 s) refrigerant compressor application. The bearings particularly hybrid angular contact ball bearings were lubricated with an ultra-low viscosity lubricant forming an ultra-thin lubrication film thickness of 200 nm or less, in particular a modern, environmental friendly refrigerant, or a refrigerant with one or more other particularly lubrication-relevant substances up to a total concentration of 1%. Thereby bearing sets of various dimensions and internal and external tolerances were investigated. These investigations were focusing on an optimized bearing life in connection with an optimized compressor operating performance. Thereby the key findings of these extensive investigations were that a long bearing life together with a good compressor operating performance is strongly influenced be the correct selection of the axial clearance of the two hybrid ball bearings and that the optimal axial clearances are dependent on the bearing size, particularly on the inner diameter of a ring-shaped inner raceway element (or short inner ring) of the bearings, respectively the ball pitch diameter e.g. for arrangements where no classic bearing inner ring exists, e.g. if the inner raceway is directly manufactured on the rotating shaft. Whereas particularly a too small axial clearance leads to high internal bearing load conditions, high contact stresses and break-throughs of the lubricant film resulting in a short bearing life. On the other hand, if the axial clearance is too big sliding motion occurs between the balls and the raceways, the contact stress are too low to cause an increase in viscosity and the lubricant film thickness is reduced, causing further wear and a short bearing life. Bearing stiffness and rotor positioning accuracy are also negatively affected by too large clearance. So, the design rules in the paragraphs above ensure a long bearing life in connection with a good compressor operating performance.

Further advantages, features and details of the invention result from the exemplary embodiments of the invention described in the following with reference to the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b show longitudinal sections through the upper half of two hybrid ball bearings, which are arranged face-to-face in FIG. 1a and back-to-back in FIG. 1 b,

FIG. 2 shows a longitudinal section through an upper region of a bearing arrangement of a refrigerant compressor, and

FIG. 3 shows a cross-section through the center of the arrangement in FIG. 2.

DETAILED DESCRIPTION

FIGS. 1a and 1b show as embodiments of the invention longitudinal sections through the upper half of two hybrid ball bearing. Thereby FIG. 1a shows a bearing arrangement of two hybrid angular contact ball bearings which are arranged to each other in a face-to-face arrangement what is also called a X-arrangement. FIG. 1b shows a bearing arrangement of two hybrid angular contact ball bearings which are arranged to each other in a back-to-back arrangement what is also called a O-arrangement. Each of the two hybrid angular contact ball bearings of FIGS. 1a and 1b comprises a ring-shaped outer raceway element 12 and 22 and an inner raceway element 11 and 21. The inner and outer raceway elements 11, 21, 12 and 22 are made of a stainless steel, particular a high nitrogen stainless steel, e.g. Chromex 40, Cronidur 30; X30CrNoN15-1, X40CrMoVN16-2, NitroMax or similar with a hardness bigger than HRC 58 at least at the raceways and a corrosion resistance with a pitting potential higher or equal to −25 mV according to ASTM G61-86. The inner raceway elements 11 and 21 have an inner diameter d respectively d′.

Series of ceramic balls 15 and 25 e.g. of Si₃N₄ are arranged for each ball bearing between the inner and outer raceway elements 11, 21, 12 and 22 of the respective ball bearing. The balls 15 and 25 are snapped and held in cages 14 and 24. In this case, the cages 14 and 24 prevent mutual contact of the balls 15 and 25 within the respective row of balls 15 and 25. The cages 14 and 24 are made of fiber-reinforced PEEK. Thereby the row of balls 15 defines a pitch diameter dm being the diameter of a circle going through the center points of all balls 15; and the row of balls 25 defines in a similar manner the pitch diameter dm′.

Each of the hybrid angular contact ball bearings shows a contact angle α respectively β. This contact angle α respectively β is the angle between a connecting line between the nominal contact points of a ball 15 or 25 with the outer and inner raceway element 12 and 11 respectively 22 and 21 on the one side and a plane of the ball bearing being perpendicular to the axial direction on the other side.

In FIGS. 1a and 1b the two hybrid angular contact ball bearings are identical particularly meaning that the inner diameters d and d′, the pitch diameters dm and dm′ and the contact angles α and β are identical. In other embodiments, particularly the contact angles α and β are different from each other e.g. to cope with different expected axial loads in the one and the other axial direction. Of course, there are specific cases where also the inner diameters d and d′ and/or the pitch diameters dm and dm′ of the two hybrid ball bearings are different from each other.

The two hybrid ball bearings are configured and manufactured for high rotation speeds between 2500 and 25000 rpm or ndm-values being between 500000 and 1200000 mm/60 s, whereby the ndm-value is the multiplication result of the rotation speed and the pitch diameter dm or dm′. Further the two hybrid ball bearings are configured and manufactured the way that they exhibit in not used, as new respectively not run in conditions a specific axial clearance C with zero measuring and zero mounting loads as explained above as the cognition of this invention to ensure optimized bearing life together with optimized compressor operating performance.

More particularly the two hybrid angular contact ball bearings have an inner diameter d=d′=150 mm, a pitch diameter dm=dm′=187.5 mm, a contact angle α=β=15° and the axial clearance C with zero measuring and zero mounting loads is around 15 μm In another embodiment, which varies from the aforementioned, the contact angles α and β of the two hybrid ball bearings are different, e.g. α=15°, β=30° and C is then around 13 μm

Thereby a negative axial clearance C has to be understood by referring to FIGS. 1a and 1b as follows: With respect to the face-to-face arrangement of FIG. 1a a negative axial clearance C means that the axial gap is no longer between the two inner raceway elements 11 and 21 which are then contacting each other, but between the two outer raceway elements 12 and 22 showing the absolute value of the negative axial clearance C. With respect to the back-to-back arrangement of FIG. 1b a negative axial clearance C means that the axial gap is no longer between the two outer raceway elements 12 and 22 which are then contacting each other, but between the two inner raceway elements 11 and 21 showing the absolute value of the negative axial clearance C.

Thereby the two hybrid angular contact ball bearings can be paired bearings, what increases the degree of freedom to ensure the desired axial clearance C; or they are unpaired what increases the demand on the tolerances on the single bearing to ensure the desired axial clearance C, but of course makes it easier from a handling and mounting perspective as no pair relationship has to be followed.

Of course, in other embodiments there can be one or more additional hybrid ball and/or roller particularly cylindrical roller bearings on the left and/or the right side of the bearings of FIGS. 1a and 1b or also in between the two bearings of FIGS. 1a and 1b depending on the required load carrying capacity at the respective installation location of the bearings and the application requirements. Thereby these additional bearings do not influence said axial clearance of the two bearings in FIGS. 1a and 1 b.

FIG. 2 shows, as an exemplary embodiment of the invention, a longitudinal section through an upper region of a bearing arrangement of a refrigerant compressor, in particular of a large centrifugal air-conditioning compressor with a direct drive, and FIG. 3 shows a cross-section through the center of the illustration of FIG. 2. The compressor comprises a housing 110, of which, in FIGS. 2 and 3, only the regions which are in connection with the bearing arrangement are shown. Furthermore, the housing 110 comprises an axial circular cylindrical bore 118 which receives the hybrid ball bearings and an outer intermediate ring 130.

In the bore 118, two hybrid angular contact ball bearings according to FIG. 1b are spaced apart from one another by the outer intermediate ring 130. The ring-shaped outer raceway elements 12 and 22 are fixed in the housing 110 in a manner known per se and not shown in detail. Also, the inner raceway elements 11 and 21 are secured in the manner not shown in the known manner on the shaft of the compressor, which is also not shown, which is provided for rotation relative to the housing 110. In this case, the shaft can, for example, merge into a rotor of an electric machine serving to drive the shaft. To the right of the illustration of FIG. 2, a blade wheel of the compressor for the refrigerant of the air-conditioning system can be arranged on the shaft, for example. Anyhow the shaft may have another bearing location distanced from the one show in FIG. 2 e.g. on the other axial side of the drive. This other bearing location can be of identical, similar, but also different design as shown in FIGS. 2 and 3.

The series of ceramic balls 15 and 25 are arranged between the outer and inner raceway elements 12, 22, 11 and 21 of the respective ball bearing. The balls 15 and 25 are respectively snapped and held in the cages 14 and 24. The cages 14 and 24 prevent mutual contact of the balls 15 and 25 within the respective row of balls 15 and 25.

The outer intermediate ring 130 is arranged between the two outer raceway elements 12 and 22 of the two hybrid angular-contact ball bearings, and an inner intermediate ring 138 is arranged between the inner rolling-surface elements 11 and 12 of the two ball bearings. The housing 110 comprises a radial circular cylindrical bore 112 into which a nozzle 140 for the refrigerant penetrating through the bore 112 can be screwed. The outer intermediate ring 130 is formed with a slot 132 aligned with the radial bore 112 and a radial, circular cylindrical penetration 134 opposite the slot 132. The radial penetration 134 corresponds in its position to a radial bore 114 provided in the housing 110, which serves as a drain for the refrigerant supplied to the bearing arrangement.

The bearing arrangement is configured for lubrication with a more or less pure, oil-free refrigerant, for example one of the modern, environmental friendly refrigerants or with a refrigerant with one or more other particularly lubrication-relevant substances up to a total concentration of 1%, as follows: The nozzle 140 passing through the bore 110 as well as the slot 132 of the intermediate ring 130 is screwed into the bore 112 of the housing 110 and secured, for example, by means of an adjusting nut 148. For this purpose, the nozzle 140 is designed, at least in its upper region, with an external thread and the bore 112, at least in its upper region, with an internal thread corresponding thereto. Furthermore, the nozzle 140 comprises a channel 144 which, on the bearing side, merges into a Y-shaped outlet channel 144. The Y-shaped design of the channel 144 is thereby of particular advantage, if for example not enough space is available for a T-shaped design.

When a corresponding refrigerant line is connected at the upper end of the nozzle 140, the refrigerant feed is then effected via the channel 142. The refrigerant is then conveyed via the Y-shaped outlet channel 144 between the lower edge of the cages 14 and 24 and the outer mantles of the inner raceway elements 11 and 21 and therewith directly injected into the rolling zones of the hybrid angular contact ball bearings. For this purpose, the ball bearings as well as the nozzle 140 are configured in a structurally and geometrically coordinated manner with respect to one another, such as the screw-in depth into the bore 112.

During operation of the compressor, the refrigerant is supplied in liquid form in saturated condition. The bearing arrangement is configured in such a way that a pressure situation for the refrigerant is maintained, which largely prevents the refrigerant from passing over into the gaseous state. In particular, the diameters of the Y-shaped outlet channel are correspondingly dimensioned. Furthermore, the bearing geometry, in particular with regard to an axial free passage possibility for the injected refrigerant, but also the geometry of the drain 114, are correspondingly designed, particularly in a limiting manner. Thereby the diameters of the channels are dimensioned such that there is a very little pressure drop upstream the Y-shaped outlet channel 144. The refrigerant passes the outlet channel 144 in liquid form. After leaving the outlet channel 144 some refrigerant is transformed into gas, depending on the pressure drop across the outlet channel 144, the bearing friction and the time it takes to flow through.

In other embodiments, the outer intermediate ring 130 can also be installed rotated by 180° so that the penetration 134 is aligned with the bore 112 and the slot 132 with the bore 114. There are also embodiments in which the slot 132 and the penetration 134 do not lie opposite to one another, but are arranged to be circumferentially staggered by 160° or 140°, for example. In further other embodiments, the slot 132 can be replaced by a further radial penetration. Furthermore, the outer intermediate ring can also comprise a plurality of penetrations for more than one nozzle and possibly also for further outflows. Alternatively, the drain 114 and an associated penetration or slot of the outer intermediate ring may be dispensed with.

In other embodiments, the inner intermediate ring 138 can support the introduction of the refrigerant into the rolling-off zones, in particular by a radially outwardly bulged configuration of its outer shell, for example in the form of a roof. In still other embodiments, the nozzle can, of course, also be fastened in another way in the housing.

In other embodiments of lubrication spacers can be configured according to DE 20 2017 106 076 U1 and CN 205 025 807 U.

Of course, it has to be emphasized that the intermediate rings 130 and 138 do not affect the desired axial clearance C with zero measuring and zero mounting loads of the bearings. In other embodiments, it also can be a one-piece intermediate ring or also more of such intermediate rings, respectively further bearings. In other embodiments, it also can be e.g. a single washer located between the inner raceway elements 11 and 21 or the outer raceway elements 12 and 22, particularly in connection with a different supply of the lubricating refrigerant, e.g. axially from at least one of the side faces. Thereby then also the washer can be included in the arrangement to control the desired axial clearance C, whereby in principle also such intermediate ring(s) can be used and configured for this. 

What is claimed is:
 1. A pair of hybrid ball bearings comprising: a first hybrid ball bearing comprising a first stainless steel inner ring having an inner diameter, a first stainless steel outer ring and a first plurality of ceramic balls between the first inner ring and the first outer ring; a second hybrid ball bearing comprising a second stainless steel inner ring having the inner diameter, a second stainless steel outer ring and a second plurality of ceramic balls between the second inner ring and the second outer ring; wherein the pair of hybrid ball bearings are configured a) such that when the first inner ring and the first outer ring are in contact with the first plurality of ceramic balls under zero measuring load and zero mounting load and the second inner ring and the second outer ring are in contact with the second plurality of ceramic balls under zero measuring load and zero mounting load, and the first inner ring is in direct contact with the second inner ring, a first gap exists between the first outer ring and the second outer ring or b) such that when the first inner ring and the first outer ring are in contact with the first plurality of ceramic balls under zero measuring load and zero mounting load and the second inner ring and the second outer ring are in contact with the second plurality of ceramic balls under zero measuring load and zero mounting load, and the first outer ring is in direct contact with the second outer ring, a second gap exists between the first inner ring and the second inner ring, wherein the first gap and the second gap are related to the inner diameter as follows: if the pair of hybrid ball bearings have an X configuration and the inner diameter is less than or equal to 80 mm, the first gap is less than 5 μm or the second gap is less than 5 μm or the second gap is between 25 μm and 40 μm; if the pair of hybrid ball bearings have the X configuration and the inner diameter is greater than 80 mm and less than or equal to 180 mm, the first gap is less than 5 μm or the second gap is less than 20 μm; if the pair of hybrid ball bearings have the X configuration and the inner diameter is greater than 180 mm, the first gap is less than 5 μm or the second gap is less than 30 μm; if the pair of hybrid ball bearings have an O configuration and the inner diameter is less than or equal to 80 mm, the second gap is less than 5 μm or the first gap is less than 5 μm or the first gap is between 25 μm and 40 μm; if the pair of hybrid ball bearings have the O configuration and the inner diameter is greater than 80 mm and less than or equal to 180 mm, the second gap is less than 5 μm or the first gap is less than 20 μm; and if the pair of hybrid ball bearings have the O configuration and the inner diameter is greater than 180 mm, the second gap is less than 5 μm or the first gap is less than 30 μm.
 2. The pair of hybrid ball bearings according to claim 1, wherein the pair of hybrid ball bearings are in a not used, as new, not run in condition and are identical.
 3. The pair of hybrid ball bearings according to claim 1, wherein the first plurality of ceramic balls are retained in a first bearing cage formed of a glass-fiber-reinforced PEEK, wherein the second plurality of ceramic balls are retained in a second bearing cage formed of the glass-fiber-reinforced PEEK, and wherein the stainless steel is a high nitrogen bearing steel.
 4. The pair of hybrid ball bearings according to claim 1, including a hollow-cylindrical lubrication spacer or distance ring between the first outer ring and the second outer ring or between the first inner ring and the second inner ring.
 5. The pair of hybrid ball bearings according to claim 1, wherein the first and second hybrid ball bearings are angular contact ball bearings with contact angles between 15 and 30°.
 6. The pair of hybrid ball bearings according to claim 5, wherein the contact angle of the first hybrid ball bearing is from 10 to 20° and the contact angle of the second hybrid ball bearing is from 21 to 35°.
 7. The pair of hybrid ball bearings according to claim 1, wherein the first and second hybrid ball bearings are configured for rotation speeds between 2500 and 25000 rpm or ndm-values between 500000 and 1200000 mm/60 s, wherein the ndm-value is the multiplication result of the rotation speed and the pitch diameter.
 8. The pair of hybrid ball bearings according to claim 1, wherein the two hybrid ball bearings are paired and form a synchronized set.
 9. A compressor comprising: a stator; a rotor supported by the pair of hybrid bearings according to claim 1 for rotation relative to the stator; and a refrigerant in contact with the first plurality of ceramic balls.
 10. A pair of hybrid ball bearings comprising: a first hybrid ball bearing comprising a first stainless steel inner ring having a pitch diameter, a first stainless steel outer ring and a first plurality of ceramic balls between the first inner ring and the first outer ring; a second hybrid ball bearing comprising a second stainless steel inner ring having the pitch diameter, a second stainless steel outer ring and a second plurality of ceramic balls between the second inner ring and the second outer ring; wherein the pair of hybrid ball bearings are configured a) such that when the first inner ring and the first outer ring are in contact with the first plurality of ceramic balls under zero measuring load and zero mounting load and the second inner ring and the second outer ring are in contact with the second plurality of ceramic balls under zero measuring load and zero mounting load, and the first inner ring is in direct contact with the second inner ring, a first gap exists between the first outer ring and the second outer ring or b) such that when the first inner ring and the first outer ring are in contact with the first plurality of ceramic balls under zero measuring load and zero mounting load and the second inner ring and the second outer ring are in contact with the second plurality of ceramic balls under zero measuring load and zero mounting load, and the first outer ring is in direct contact with the second outer ring, a second gap exists between the first inner ring and the second inner ring, wherein the first gap or the second gap is related to the pitch diameter as follows: if the pair of hybrid ball bearings have an X configuration and the pitch diameter is less than or equal to 100 mm, the first gap is less than 5 μm or the second gap is less than 5 μm or the second gap is between 25 μm and 40 μm; if the pair of hybrid ball bearings have the X configuration and the pitch diameter is greater than 100 mm and less than or equal to 200 mm, the first gap is less than 5 μm or the second gap is less than 20 μm; if the pair of hybrid ball bearings have the X configuration and the pitch diameter is greater than 200 mm, the first gap is less than 5 μm or the second gap is less than 30 μm; if the pair of hybrid ball bearings have an O configuration and the pitch diameter is less than or equal to 100 mm, the second gap is less than 5 μm or the first gap is less than 5 μm or the first gap is between 25 μm and 40 μm; if the pair of hybrid ball bearings have the O configuration and the pitch diameter is greater than 100 mm and less than or equal to 200 mm, the second gap is less than 5 μm or the first gap is less than 20 μm; and if the pair of hybrid ball bearings have the O configuration and the pitch diameter is greater than 200 mm, the second gap is less than 5 μm or the first gap is less than 30 μm.
 11. The pair of hybrid ball bearings according to claim 10, wherein the pair of hybrid ball bearings are in a not used, as new, not run in condition and are identical.
 12. The pair of hybrid ball bearings according to claim 10, wherein the first plurality of ceramic balls are retained in a first bearing cage formed of a glass-fiber-reinforced PEEK, wherein the second plurality of ceramic balls are retained in a second bearing cage formed of the glass-fiber-reinforced PEEK, and wherein the stainless steel is a high nitrogen bearing steel.
 13. The pair of hybrid ball bearings according to claim 10, including a hollow-cylindrical lubrication spacer or distance ring between the first outer ring and the second outer ring or between the first inner ring and the second inner ring.
 14. The pair of hybrid ball bearings according to claim 10, wherein the first and second hybrid ball bearings are angular contact ball bearings with contact angles between 15 and 30°.
 15. The pair of hybrid ball bearings according to claim 14, wherein the contact angle of the first hybrid ball bearing is from 10 to 20° and the contact angle of the second hybrid ball bearing is from 21 to 35°.
 16. The pair of hybrid ball bearings according to claim 10, wherein the first and second hybrid ball bearings are configured for rotation speeds between 2500 and 25000 rpm or ndm-values between 500000 and 1200000 mm/60 s, wherein the ndm-value is the multiplication result of the rotation speed and the pitch diameter.
 17. The pair of hybrid ball bearings according to claim 10, wherein the two hybrid ball bearings are paired and form a synchronized set.
 18. A compressor comprising: a stator; a rotor supported by the pair of hybrid bearings according to claim 10 for rotation relative to the stator; and a refrigerant in contact with the first plurality of ceramic balls. 